Antenna Matchbox Evolution
A "simple" name with dozens of meanings!
Let's take a look at the evolution of the antenna matchbox
("antenna tuner" - if you prefer) evolution over the past 80 years:
60 years ago, the very best way of matching an openwire-fed doublet was to use a balanced, specifically “link-coupled” antenna matchbox. The champion of the time was the Johnson Viking Matchbox.
In the meantime, the name "antenna matchbox" has been superseded by “antenna tuner”, but I will use the acronym ATU (for short), meaning “antenna tuning unit”, below.
The goal is to match the 50 Ohm unbalanced output of the transceiver to the varying balanced load of the “antenna system”. The antenna system includes the antenna, the feedline all the way to the shack, and anything else that might be in the line before connecting to the ATU.
In the case of an openwire-fed doublet, this can be anything from a few Ohms, up to about 3k Ohms. The impedance of the feedpoint is transformed along the feedline, causing it to be different at different places along the feedline. This is due to Transmission Line Transformation (TLT).
Before the 1950’s we had only 4 HF ham bands – 80/40/20/10m. We were allocated the 15m band in 1952. There were a few symmetrical ATUs on the market at the time. These had link coupling.
The most popular was the Johnson Viking Matchbox, in 1kW and 274W versions. You can find a description of most of these on my web site on the following two pages:
- https://www.dj0ip.de/antenna-matchboxes/symmetrical-matchboxes/low-power-atu-s/
- https://www.dj0ip.de/antenna-matchboxes/symmetrical-matchboxes/high-power-atu-s/
These were relatively expensive because they required complex switching and complex “dual-differential” variable capacitors.
Following WW-II, 50Ω and 70Ω coax began replacing openwire as the feedline of choice amongst hams.
Multi-band dipole antennas such as the W3DZZ and G5RV were popular, and hams had no problem matching them with the Pi-Network in the tube-transmitters at the time.
As SSB rapidly replaced AM, transceivers replaced the old transmitter/receiver station. These typically had much smaller components in their Pi-Network, thus having a much smaller impedance matching range as the older, much larger transmitters. The need for an unbalanced ATU emerged.
In July of 1970, Lew McCoy (W1ICP) published an article in QST describing "The Ultimate Transmatch". This was an all-band ATU using a complex circuit, similar to today’s popular T-Network ATUs. Soon several commercial versions of this ATU appeared on the commercial market, albeit using a simplified version, with a roller inductor and two (simple) variable capacitors. This soon became the hobby’s favorite ATU and remained so for the next 30 years – followed by automatic tuners. Almost all of the auto-ATUs were also unbalanced.
In the early 1980s, amateur radio was allocated 3 additional HF ham bands, known as the WARC bands; 30m/17m/12. This brought the total number of HF ham bands to 8. If we include 160m, which is just below the HF spectrum at the top of the Medium Wave spectrum, thus known as "Top Band", we have 9 ham bands of interest.
Trying to cover 9 ham bands with one or two antennas is quite a challenge for a coax-fed antenna. As a result, the decades old openwire-fed-dipole, often called “doublet” re-emerged as a popular all band antenna. Note: Old-Timers are annoyed by calling these antennas "doublets", as we had previously called our dipole antennas “doublets”.
In the meantime, there were no more truly symmetrical (balanced) ATUs commercially available.
A quick fix was found by placing a 4:1 Ruthroff (Voltage) balun between the unbalanced ATU and the balanced openwire-feedline. This generally worked “OK” as long as the antenna was a half-wavelength long and the ATU had a broad impedance matching range – which most manual ATUs had.
In 1992, Roy LeWallen’s (W7EL) excellent articles entitled Baluns: What They Do And How They Do It
was published in the “ARRL ANTENNA COMPENDIUM VOLUME ONE”.
In this document, Roy explained how THE primary task of a balun was to impede the flow of Common Mode Current (CMC) on the outer surface of the coaxial shield. The ham community knew that a balun was used for connecting unbalanced coax to a balanced antenna, but most of us did not really understand what it did, or how it did it.
Roy also introduced two new terms: “Voltage Balun” and “Current Balun”, renaming the “Ruthroff” and “Guanella” baluns. Roy explained how the current (Guanella) balun had much more Common Mode Impedance (CMI) than the voltage (Ruthroff) balun.
This document caused a paradigm shift in balun technology, with the current balun rapidly replacing the voltage balun as the balun of choice in most HF antenna and ATU applications.
The evolution continues...
The Internet and advances in telecommunications introduced an invasion of consumer products to our households. These devices, especially some of the imports, were highly susceptible to radio frequency interference (RFI). Knowledgeable hams such as (alphabetical order) Owen Duffy - VK1OD, Steve Hunt – G3TXQ (SK), and Tom Rauch – W8JI taught us that the 1:1 current balun was significantly more efficient at reducing CMC than the 4:1 current balun, for two reasons:
- Using like-Toroids, the 1:1 balun typically has twice as much CMI as the 4:1 balun, and
- The 4:1 balun’s ability to impede CMC rapidly decreases as SWR increases, whereas the 1:1 balun does not; at least not nearly as much.
Since the antenna system’s impedance can be all over the map, depending on the length of the antenna and length of the feedline, the impedance is rarely near the 200/400 Ohm impedance of the output of the 4:1 balun when it is using coax/bifilar transmission lines. Thus the SWR rises, decreasing the effectiveness of the 4:1 balun, whereas the 1:1 balun continues to have good CMI.
Further, in the event that the antenna system’s impedance is low, which happens often, the 4:1 balun will transform its impedance 4x lower, presenting a very low impedance to the ATU. ATUs tend to have their lowest efficiency and highest loss of power at very low impedances.
As a result, most high-end ATU manufacturers now use 1:1 current baluns in their flagship products. Unfortunately, this improvement has not yet been implemented in many of the mid-range and entry-level ATUs.
So how well does this unbalanced-ATU/1:1 Current balun combination work?
Several hams have conducted tests comparing this combination to balanced ATUs (i.e., the Johnson Viking Matchbox, or the Annecke Symmetrical Coupler). These include DF2BC, KD7MW, W8JI, and WB6BYU. You can read about these tests (I call them "Shootouts"), here:
https://www.dj0ip.com/ant-tun-sh-outs
After studying the vast amount of information in these 4 shootouts, we can only conclude that this method (unbalanced ATU + 1:1 current balun) works very well. In most cases the results showing the “amount of” and “balance of” RF current in the two wires of the feedline typically matches or exceeds that of the older balanced (link-coupled) technology.
Further references are found at the links shown at the bottom of this page: